1 Tech Note 0001 2 How to Gather Entropy on Embedded Systems 3 Tom St Denis 4 5 Introduction 6 ------------ 7 8 This tech note explains a relatively simple way to gather entropy for a PRNG (Yarrow in this case) in embedded systems 9 where there are few sources of entropy or physical sources. 10 11 When trying to setup a secure random number generator a fresh source of random data (entropy) is required to ensure the 12 deterministic state of the PRNG is not known or predetermined with respect to an attacker. 13 14 At the very least the system requires one timer and one source of un-timed interrupts. by "un-timed" I mean interrupts 15 that do not occur at regular intervals [e.g. joypad/keypad input, network packets, etc...]. 16 17 First we shall begin by taking an overview of how the Yarrow PRNG works within libtomcrypt. At the heart of all 18 PRNGs is the "prng_state" data type. This is a union of structures that hold the PRNG state for the various prngs. The 19 first thing we require is a state... 20 21 prng_state myPrng; 22 23 Next we must initialize the state once to get the ball rolling 24 25 if (yarrow_start(&myPrng) != CRYPT_OK) { 26 // error should never happen! 27 } 28 29 At this point the PRNG is ready to accept fresh entropy which is added with 30 31 int yarrow_add_entropy(const unsigned char *buf, unsigned long len, prng_state *prng) 32 33 This function is **NOT** thread safe which will come under consideration later. To add entropy to our PRNG we must 34 call this function with fresh data as its sampled. Lets say we have a timer counter called "uTimer" which is a 32-bit 35 long and say a 32-bit joyPad state called "uPad". An example interrupt handler would look like 36 37 void joypad_interrupt(...) { 38 unsigned char buf[8]; 39 40 STORE32L(uTimer, buf); 41 STORE32L(uPad, buf+4) 42 if (yarrow_add_entropy(buf, 8, &myPrng) != CRYPT_OK) { 43 // this should never occur either unless you didn't call yarrow_start 44 } 45 46 // handle interrupt 47 } 48 49 In this snippet the timer count and state of the joypad are added together into the entropy pool. The timer is important 50 because with respect to the joypad it is a good source of entropy (on its own its not). For example, the probability of 51 the user pushing the up arrow is fairly high, but at a specific time is not. 52 53 This method doesn't gather alot of entropy and has to be used to for quite a while. One way to speed it up is to tap 54 multiple sources. If you have a network adapter and other sources of events (keyboard, mouse, etc...) trapping their 55 data is ideal as well. Its important to gather the timer along with the event data. 56 57 As mentioned the "yarrow_add_entropy()" function is not thread safe. If your system allows interrupt handlers to be 58 interrupted themselves then you could have trouble. One simple way is to detect when an interrupt is in progress and 59 simply not add entropy during the call (jump over the yarrow_add_entropy() call) 60 61 Once you feel that there has been enough entropy added to the pool then within a single thread you can call 62 63 int yarrow_ready(prng_state *prng) 64 65 Now the PRNG is ready to read via the 66 67 unsigned long yarrow_read(unsigned char *buf, unsigned long len, prng_state *prng) 68 69 It is a very good idea that once you call the yarrow_ready() function that you stop harvesting entropy in your interrupt 70 functions. This will free up alot of CPU time. Also one more final note. The yarrow_read() function is not thread 71 safe either. This means if you have multiple threads or processes that read from it you will have to add your own semaphores 72 around calls to it. 73 74
